Organic carbon accumulation events in the mid-Cretaceous rocks of the Pieniny Klippen Belt (Polish Carpathians) -- a petrological and geochemical approach

(1)

Organic carbon accumulation events in the mid-Cretaceous rocks of the Pieniny Klippen Belt (Polish Carpathians) — a petrological

and geochemical approach

Patrycja WÓJCIK-TABOL

Wójcik-Tabol P. (2006) — Organic carbon accumulation events in the mid-Cretaceous rocks of the Pieniny Klippen Belt (Polish Carpathians) — a petrological and geochemical approach. Geol. Quart., 50 (4): 419–436. Warszawa.

New petrological and geochemical data lead to a consistent depositional model of the Corg.-rich sedimentation within the Pieniny Basin during the mid-Cretaceous. Considerable terrestrial runoff into the Pieniny Basin occurred during the late Albian. Detrital macerals accu- mulated under aerobic conditions on the shelf and continental slope. Fertilization of surface water induced primary productivity; aerobic degradation of organic matter led to the development of an oxygen-minimum zone within mid-water. The oxygen-minimum zone spread over almost all of the Pieniny Basin (Albian/Cenomanian). At the same time, a stagnant pool developed in the Grajcarek Basin. During the mid-Cenomanian the O2minimum zone retracted and covered only the shelf and upper/middle slope. Stagnant pools might have formed in the depressions. Turbidity currents flowed down the slope and deposited calciturbiditic sequences with organic detritus in the Branisko and Pieniny basins. At the end of the Cenomanian, isolated anoxic or even H2S-bearing basins existed on the shelf. The slope was still occupied by the oxygen-minimum zone. In the deepest part of the sea-floor a stagnant basin formed.

P. Wójcik-Tabol, Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, PL-30-063 Kraków, Poland, e-mail:

woj@ing.uj.edu.pl (received: August 1, 2005; accepted: June 05, 2006).

Key words: Pieniny Klippen Belt, mid-Cretaceous, oceanic anoxic events, black shales, geochemical indicators.

INTRODUCTION

Black shale sedimentation was widespread in Europe and other parts of the world during the Cretaceous, from the Aptian until the Turonian in particular (eg. the Pacific: Thiede et al., 1982; the North Europe: Jenkyns, 1985; the South Europe: Ar- thur and Premoli Silva, 1982; Baudin et al., 1998; the Atlantic:

Brumsack, 1980, 1986; Venezuela: Alberdi-Genolet and Tocco, 1999) and peaked during global Oceanic Anoxic Events (OAEs, cf. Schlanger and Jenkyns, 1976). Develop- ment of anoxic facies in black shales was explained using three models: detrital, productivity and stagnant anoxic. The “Detri- tal Anoxic Event” model suggests that supply of terrestrial organic matter and sedimentation rate are high to produce the anoxity (Erbacher and Thurow, 1990). The “Productivity Anoxic Event” model suggests that nutrient input (by upwelling or by river) leads to elevated surface water productivity (Erbacher and Thurow, 1995; Hay, 1995). The oxygen used to oxidise organic remains led to development of a mid-water oxygen-minimum zone. If oxygen-minimum zone

grew larger, there would develop a “Stagnant Anoxic Event”

(Brumsack and Thurow, 1986; Erbacher and Thurow, 1995), due to stagnant or sluggish water circulation (op. cit.).

The extreme warming in the mid part of the Cretaceous represents one of the best examples of “greenhouse” climate con- ditions in the geological record (De Graciansky et al., 1987;

Hoefs, 1997). Globally averaged surface temperatures in the mid-Cretaceous were over 10°C higher than today (De Boer, 1982). The oceans of the Cretaceous may have been similar to the modern Pacific Ocean, both with large marginal seas (Chamley and Robert, 1982; Hay, 1995). It is possible that the Tethys was a hypersaline area, where water became more saline and dense because of evaporation. Meridional seaways that connected regions with surface waters having similar density but different temperatures and salinities may have played a spe- cial role in the Cretaceous oceans (Hay, 1995, 1997).

Episodes of increased accumulation of organic matter in the Cretaceous deposits coincide with transgression periods (Schlanger and Cita, 1982). The oxygen content of sea-water was low and the water circulation was sluggish during the greenhouse period (Hay, 1995). As a result, remineralization of

(2)

organic matter and thus recycling of nutrients was hampered.

Moreover, an increased loss of nitrate caused by denitrification in oxygen deficient waters might have led to “starved basin”

development (Brumsack, 1980; Brumsack and Thurow, 1986).

Black shale deposition in the Central Atlantic and western Tethys was attributed to stagnant pools of warm saline bottom wa- ter (Brass et al., 1982). It may also have been caused by O2deple- tion, being the response to periodic development of a positive fresh-water balance (large land runoff, low evaporation, flow of nutrient-rich surface water out to the open ocean) and estuarine circulation (Rohling and Hilgen, 1991). Additionally, increasing runoff from land brought nutrients and fertilized the oceans. This is indicated by the widespread occurrence of siliciclastics along the margins of the western Tethys (Weissert, 1990).

The Cretaceous rock formations of the Pieniny Klippen Belt (PKB) have been the focus of scientific interest for many years, with research into their lithology, palaeontol- ogy/biostratigraphy and sedimentology (Birkenmajer, 1960;

1977, 1979; Aleksandrowicz et al., 1968; Sikora, 1971;

Ksi¹¿kiewicz, 1972; Gasiñski, 1988). The layers of black shale intercalated within the Cretaceous strata of the PKB have been interpreted by Birkenmajer and Jednorowska (1984, 1987) and Gasiñski (1988, 1991) as records of anoxic events. These au- thors suggested that local oxygen deficiency episodes are equivalents of the global Oceanic Anoxic Events, as, during Mesozoic time, the Pieniny Basin belonged to the northern branch of the Tethys Ocean.

This paper presents the results of a multidisciplinary approach that combines sedimentological, petrological and geochemical methods in order to propose a consistent depositional model for the mid-Cretaceous organic carbon-rich (Corg.-rich) sediments within the Pieniny Basin.

GEOLOGICAL SETTING

The Pieniny Klippen Belt (PKB) represents a long and nar- row arch-like structure situated in the Palaeo-Alpine Accretio- nary wedge between the Inner and the Outer Carpathians (Fig. 1; Birkenmajer, 1979).

The facies division distinguishable between Jurassic sequences points to various depositional realms. The shallow, threshold-like one is called the Czorsztyn Succession. This developed on an elevation of the northern ridge (the Czorsztyn Ridge) of the Pieniny Basin. The Niedzica and Branisko successions were formed on the southern slope of the Czorsztyn Ridge. The deepest part of the PKB Basin was occupied by deep water facies of the Pieniny Succession (Fig. 2;

Birkenmajer, 1977).

Unification of the lithofacies occurred there during mid-Cretaceous time. Deposition of marls and marly shales dominated across the entire Pieniny Basin. This model of bathymetry has been based on the foraminiferal assemblage distribution (Gasiñski, 1991; Birkenmajer and Gasiñski, 1992).

Cretaceous oceanic anoxic event records have been recorded within the Pieniny rock successions as grey and brown- ish-black, organic carbon-rich sediments. The Aptian-Albian formations: Kapuœnica, Pomiedznik and Wronine, comprise

marly shales with occasional sandstone intercalations. The Jaworki Formation is dated as Cenomanian–Turonian. It includes several lithostratigraphic units (Figs. 2and3) developed mainly as pelagic marls (i.e.: Skalski Member), shales (Magierowa Member), and calciturbidite deposits (Macelowa, Trawne, Snenica members). Planktic foraminiferal biostratigraphy enables their assignation to the following zones: the Rotalipora subticinensis–R. ticinensis, Planomalina buxtorfi–R. appenninica, R. reicheli–R. greenhornensis and the latest R. cushmani up to the Early Helvetoglobotruncana helvetica zones (after Gasiñski, 1983, 1988; Birenmajer and Jednorowska, 1984,Fig. 2).

METHODOLOGY AND MATERIALS

The Wronine Formation, characteristic of the Grajcarek Suc- cession, occurs in the following exposures: the Szczawnica RzeŸnia (the samples signed RZ) and the Sztolnia Stream (SZT).

The Pomiedznik Formation, typical of the Czorsztyn Succes- sion, occurs on the right bank of the Grajcarek River, below the church in the village of Jaworki (JK). The archival material collected from the Halka Klippe (HL) — today flooded under water of the Czorsztyn Lake — has been also studied. The Kapuœnica Formation (the Branisko and Niedzica successions) were exposed and sampled on the left slope of the Dunajec River, be- neath the dam (KP), in the Zawiasy Klippe in Kroœcienko (ZAW) and in the Grajcarek Stream in Jaworki (JG). The Trawne Member — a biostratigraphical analogue of the upper part of the Kapuœnica Fm. was sampled within the Branisko Suc- cession, in the Trawne Stream (TR 1) and in the Pasieczny Stream (PA) profiles. The Skalski Member is exposed along the road from Jaworki through to Bukowiny Hill (BUK, Niedzica Succession) and in the Trawne Stream (TR 2–5, Branisko Suc- cession). The Snenica Member from the Niedzica Succession was sampled in the Skalski Stream in Jaworki (PSK samples).

The Snenica Member occurs also within the Pieniny Succes- sion (the Macelowa Mount section — MC). The Magierowa Member (MG) recognized only within the Pieniny Succession, was collected in the stratotype profile: the Magierowa Klippe in the hamlet of Sromowce Ni¿ne.

Microstructures were determined in 55 samples using micro- scopic observation (Nikon ECLIPSE, E 600 POL; transmitted and reflected light). Organic petrology was studied in polished thin section (30 samples) under fluorescent light (Olympus OPl 3, Fl Mk II). The quality of organic matter was evaluated in 36 samples using Rock-Eval pyrolysis. Other Rock-Eval parame- ters such as the Hydrocarbon Index (HI) and Tmaxwere defined in Espitalie (1985). Trace-element contents were measured in 28 samples using ICP-OES spectrometry and INAA analysis (Ag, Al, Ca, Cd, Cu, Mn, Mo, Ni, Pb, V, Y, Zn, As, Ba, Co, Cr, Fe, Na, Th, U, W, La, Ce). The pulverized material was analyzed by ICP-OES after dissolution in a HCl–HNO3–HClO4–HF solu- tion. Trace-element contents were shale-normalized to give an estimation of their relative enrichment. The used enrichment factor is (Wedepohl, 1970, 1991):

(trace-element content/Al content)sample / (trace-element content /Al content)shale

(3)

Isotopic determinations (organic and anorganic carbon) were carried out on a MI-1305 mass spectrometer with detect- ing system. The stable isotope composition was expressed as d¹³C values relative to the PDB standard.

RESULTS AND DISCUSSION

The absolute concentrations of the selected major and trace elements are shown both on the table (Table 1) and on the graph (Fig. 4A, B). They are compared to the values of the enrichment factor. Most of the samples are sulphur-depleted;

some S concentrations are very low. The samples investigated were collected from surface outcrops. Due to weathering pro- cesses, the sulphur content was probably drastically reduced.

Most sulphides (FeS2) have been altered to oxides.

THE ALBIAN POMIEDZNIK AND KAPUŒNICA FORMATIONS

The oldest event of organic matter deposition studied is biostratigraphicaly dated as late Albian (OAE 1c; Rotalipora subticinensis–R. ticinensis LCRZ, after Gasiñski, 1988). This was investigated within the Pomiedznik Fm. (JK samples) and the Kapuœnica Fm. (KP) of the Czorsztyn and Branisko successions, respectively, and additionally within the Trawne Member (samples TR 1, PA 2) of the Branisko Succession (Fig. 3).

The JK samples from the Pomiedznik Fm. represent hemipelagic sediments — marls and marly shales. These green rocks contain black blots and intercalations of black marls. The Kapuœnica Fm. consists of black and dark grey shales with intercalations of cherty limestone and of fine-grained sandstone (Birkenmajer, 1979). The microfauna assemblages contain oligotaxic planktonic foraminifers and radiolarians coexisting with calcareous benthos (Fig. 5A; Gasiñski, 1988). The lower

Fig. 1. Location of the investigated Polish part of the Pieniny Klippen Belt (A) against the schematic geological map of the Carpathians (after ¯ytkoet al., 1989; Birkenmajer, 1979, modified)

Location of studied sections containing black shales: BUK — Bukowiny Hill, BW — Bia³a Woda Valley, HL — Halka Klippe, JG — Jaworki Grajcarek and Jaworki Church, KP — Niedzica Water Dam, MC — Macelowa Mount, MG — Magierowa Rock, PA — Pasieczny Stream, PSK — Skalski Stream, RZ — Szczawnica RzeŸnia, SZT — Sztolnia Stream, TR — Trawne Stream, ZAW — Zawiasy Klippe

(4)

Fig.2.CorrelationoftheUpperCretaceouslithostratigraphicunitsinthePieninyKlippenBeltBasin,Poland(lithostratigraphyafterBirkenmajerandJednorowska,1987;biostratigraphyafter Gasiñski,1983,1988andCaronetal.,2006;palinspasticreconstructionafterBirkenmajer,1979modified) Successions:P—Pieniny,Br—Branisko,N—Niedzica,Cz—Czorsztyn,G—Grajcarek;W—watercolumn,B—basement;forotherexplanationsseeFigure1

(5)

Fig.3.Lithostratigraphiccolumnsofstudiedsectionsandpositionofcollectedsamples(arrowed)andhorizonsofblackshales(OAE)*—afterBirkenmajerandMyczyñski(1977) ForotherexplanationsseeFigure1

(6)

SampleAgCdCuMnMoNiPbZnAlCaVYAsBaCoCrFeSThULa U/ThV/V+Ni [ppm][%][ppm][%][ppm] OAE1c

JK2<0.30.544717.1<161.814.869.72.821.462.4391492017711.860.076.62.2340.330.50 enrich.factor12.163.093.242.872.131.9243.081.523.016.325.012.832.491.211.92.172.691.140.35 KPO’<0.30.631.2272.5<149.516.999.30.619.533.517.61220005421.320.151.42.5101.790.40 enrich.factor68.0810.235.7410.711.3312.74183.23.86.3324.2350.863.886.884.021.8811.523.696.130.26 KP1<0.3<0.343.1550.6<160.922.455.73.115.864.327.8424010611.980.246.21.5360.240.51 enrich.factor2.732.252.562.911.3828.731.411.941.671.181.51.931.171.611.342.570.830.36 TR1<0.3<0.353.5947.9575.921.1105.4112139.122.423490191144.270.8212.93420.230.65 enrich.factor0.961.092.010.90.770.741.020.860.442.670.680.81.020.710.940.750.840.800.49 PA2<0.30.438.2825.6<193.714.976.23.514.388.8237.61500191063.070.158.82.8310.320.49 enrich.factor7.782.152.983.481.711.6823.031.731.422.756.542.532.981.62.022.211.961.090.33 OAE1d1

HI5<0.3<0.334.8497.1<1104.627.7612.121.25219.873240061543.542.024.63.1340.670.33 enrich.factor3.262.996.485.312.2456.911.692.0443.9517.4413.532.533.081.764.083.582.320.21 JG1<0.3<0.3192.1438.829229.580.85.62.7132.424.13.9310241133.940.0512.72.5740.200.59 enrich.factor6.750.991.582.142.121.112.721.610.930.880.8421.981.291.821.232.920.680.43 JG1’<0.3<0.31001573.5<173.516.564.14.411.374.330.21.425017754.450.0310.11.8790.180.50 enrich.factor4.474.522.171.511.1214.481.151.480.40.871.81.681.851.851.133.970.610.35 JG20.84.894.31049.1<1210.757.5499.12.916.9185.328.425.411501651143.031.436.25.4580.870.47 enrich.factor34.9112.686.44.579.467.9813.2432.854.352.1111.076.0526.53.871.911.725.154.432.990.31 JG2’<0.3<0.3621631.1<143.217.343.13.218.165.622.83.227014521.980.166.31.6530.250.60 enrich.factor3.816.441.762.171.0431.881.41.541.261.292.041.61.131.581.383.660.870.44 JG1G<0.3<0.369.11273.8<163.129.777.13.219.672.22413.4220196020.396.62.3600.350.53 enrich.factor4.255.032.573.731.8534.531.541.625.291.052.771.841.141.661.994.151.200.37 ZAW30.9<0.359324.1<173.818.8109.73.616.981.933.741.8100013872.330.057.43.4440.460.53 enrich.factor31.63.221.142.672.12.3526.461.552.0214.684.241.682.381.181.652.612.71.580.37 SZT9<0.3<0.363.61007.16.585.138.3134.67.35.4119.821.949.634022874.181.1210.62.7370.250.58 enrich.factor1.711.743.941.522.111.424.171.120.658.590.711.41.171.051.171.021.120.870.42 RZ1<0.3<0.337.51723.7<153.721.7112.35.37.3104.420.326.354015853.560.6711.73.5370.300.66 enrich.factor1.394.111.321.651.637.761.340.836.271.551.321.581.231.781.831.541.030.50 Table1 ChemicalcompositionofthePieninysamples

(7)

Tab.1continued SampleAgCdCuMnMoNiPbZnAlCaVYAsBaCoCrFeSThULa U/ThV/V+NiV/Cr [ppm][%][ppm][%][ppm] OAE1d2

BUK2R<0.3<0.323920.2<144.317.447.22.325.655.424.33.72408552.350.0351240.200.561.01 enrich.factor1.975.062.513.041.5862.741.642.282.031.591.622.351.871.751.22.310.690.400.70 BUK3B1.51.2104.3602.6<150.8123.468.92.821.8157.519.913.176020702.10.235.84250.690.762.25 enrich.factor67.729.187.332.722.3617.731.8943.893.831.535.924.143.332.461.371.673.951.982.370.621.56 BUK3G<0.3<0.387.6737.1<13815.8552.323.39623.72.62509791.790.035.61.6270.290.721.22 enrich.factor7.494.052.152.761.8457.12.842.221.431.661.823.381.421.961.922.60.980.570.84 PSK10.3<0.330.4462<147.820.4683.714.77919.15.241011802.290.059.83270.310.620.99 enrich.factor10.31.621.581.682.221.4122.41.451.111.781.691.382.131.132.132.241.611.050.460.68 BW2<0.3<0.366.3472.3<153.644.489.85.113.888.421.86.135013872.530.049.63.8320.400.621.02 enrich.factor2.561.171.373.51.3615.251.180.921.511.051.191.680.911.512.061.391.360.460.70 TR2<0.3<0.336.2682<161.422.550.211.415.284.123.84.726010913.330.027.51.5390.200.580.92 enrich.factor0.620.760.70.790.347.520.50.450.520.350.410.780.530.530.360.760.680.420.64 TR5<0.3<0.336.9487.6<157.619.983.55.311.7112.524.31541022943.090.1810.72.6410.240.661.20 enrich.factor1.371.161.411.511.2112.441.450.993.581.181.931.741.071.621.361.710.840.510.83 PA4<0.3<0.318.2363.2<1321354.94.28.978.22021.746013793.390.02113.4370.310.710.99 enrich.factor0.851.090.991.251.0111.941.271.036.531.671.441.851.482.112.241.951.060.560.69 MC2<0.3<0.329.8513.3<151.721.566.85.110.789.120.712.813013832.870.04103.5300.350.631.07 enrich.factor1.151.271.321.701.01111.831.190.883.170.391.191.61.03S1.581.91.31.200.470.7433 OAE2

MG2B0.41.883318.44168.526.3398.27.70.9483.321.333.3560271653.70.0510.74.3350.400.742.93 enrich.factor6.5715.912.120.522.32.851.373.980.664.270.65.471.111.632.110.881.121.541.011.380.602.02 MG2G<0.3<0.3104.3330.8<1159.917.1130.27.61.4128.619.931580351313.590.0212.23.1370.250.450.98 enrich.factor2.70.552.740.911.321.041.150.575.161.162.151.690.861.291.131.080.880.300.68 MG10B0.5<0.3127.6127.64.6120.329.2241.76.20.3324.818.340.8490182003.960.0311.75660.430.731.62 enrich.factor10.24.050.263.282.531.8930.273.570.648.321.211.353.171.171.522.232.361.470.591.13 MG10G<0.30.899.8204<186.413.91406.30.2136.516.433410171333.140.01112.9640.260.611.03 enrich.factor8.643.120.411.780.891.710.181.480.566.620.991.262.080.911.41.272.250.910.450.71 MG250.95.9154.2291.13.8105.816.96115.90.9466.616.828530111733.410.1411.34.2330.370.822.70 enrich.factor19.368.085.140.622.852.331.157.970.865.380.615.941.370.872.881.061.541.971.241.280.701.87 Absolutecontentsoftraceandmajorelementsandvaluesofenrichmentfactorsestimatedas:(trace-elementcontent/Alcontent)sample/(trace-elementcontent/Alcontent)shale(accordingtoWedepohl,1970,1991)

(8)

part of the Trawne Member is represented by turbiditic deposits consisting of calcareous sandstones, intercalated with ash-grey shales. A predominance of agglutinated foraminifers was observed in the turbidite samples. Representatives of Hedbergella and Rotalipora are dominant among the planktonic foraminifers (Gasiñski, 1983).

ORGANIC MATTER AND INORGANIC GEOCHEMISTRY OF THE ALBIAN STRATA

The total organic carbon (TOC) content varies between 0.3% in pale grey PA samples and 3.1% recorded in the brown- ish-black KP 0 sample (Table 2). The organic matter is mature as shown by the HI (35–118) and Tmaxvalues (about 435°C). A plot of HI versus OI measured by Rock-Eval pyrolysis (Fig. 6) points to a predominantly III/IV kerogen type (vitrinite and inertinite).

The highest TOC values, correlating with maximal HI in KP samples, indicates the presence of II type kerogen. Petrographic analyses of the macerals correspond with the pyrolysis data and show a dominance of macerals belonging to the vitrinite and inertinite groups: detrovitrinite, collinite and inertodetrinite; the occurrence of type II kerogen (bituminite in KP 0 and sporinite in the PA 2) was recognized.

The d¹³C values are –18.43 and –24.23‰ (Table 3;Fig. 8). Organic carbon significantly enriched in¹²C associated associated with land plants (Galimov, 1980, 1985). During coalification of organic remains, the isotopic composition of kerogen can be altered. Humus forming by combus- tion of higher plant remains is characterized by enrichment in¹³C (Galimov, 1985; Dean et al., 1986). The results described above in- dicate terrestrial derivation of organic matter (in TR 1 and PA 2 samples). However, the presence of bituminite-macerals of algal ori- gin (in KP samples) is possible.

The upper Albian samples are usually poor in trace elements, but they contain high amounts of calcium (Table 1;Fig. 4A). The JK 2 sample collected in the Czorsztyn Suc- cession contains notable amounts of Cd, Y and Ba. Significant concentrations of Ba characterize the PA 2 and KP 0 samples from the Branisko Succession. It is interest- ing that the enrichment factor estimated for most of the elements exceeds 1 (Table 1;

Fig. 4B). This is the consequence of lower aluminium concentrations in the samples (3% in JK 2 and 0.6% in KP 0) than in the

“average shale” standard (8.85%), with which the enrichment factor was gauged.

Fig. 4. A — correlation of absolute content of selected trace-elements (Cu, Ni, Zn, V) and V/V+Ni, V/Cr, U/Th, with absolute TOC content in studied samples; B — normalized con- centrations of the selected trace elements (Cu, Ni, Zn, V, Ba) in the Pieniny samples

(9)

The TR 1 sample differs from the others from OAE 1c. It contains 11% of aluminium and a negligible amount of calcium, hence the values of the enrichment factor exceed 1 for As, Cr, Mn and Mo only (Table 1).

Barium together with cadmium are frequently referred as proxies for palaeoproductivity (Dymond et al., 1992;

Alberdi-Genolet and Tocco, 1999). This seems to be a reason- able explanation for the enrichment of the KP 0 sample, which includes amorphous organic matter, identified as bituminite/collinite (II/III kerogen). The higher amounts of U, As and Ag measured for KP 0 have been interpreted as a palaeoproductivity indicator as well. Silver behaves as a nutrient-like element. The high Ag concentration may result from its involvement in the biogeochemical cycle of silica (Ndung’u et al., 2001). Arsenic and uranium can be bound to organic matter, especially when absorbed by algae (Mangini and Dominik, 1979). These geochemical indicators find con- firmation in the microfaunal assemblage. The abundance of radiolarians could be fossil evidence of bioproductivity. But

this could also be due to a more oceanic setting. The absence of bathypelagic foraminifera and rarity of benthos, represented mainly by agglutinated species, may indicate the appearance of mid-water oxygen-minimum zone. Low values of absolute and shale-normalized V/V+Ni and V/Cr ratios and marked Mn-enrichment suggest an oxygenated environment (Table 1;Fig. 4A; Lewan and Maynard, 1982; Lewan, 1984;

Quinby-Hunt and Wilde, 1994).

The JK 2 and PA 2 samples are similar to each other lithologically and geochemically. They both represent marls including abundant foraminiferal associations (Fig. 5D; plankton and benthos); they have the same proportions of Al to Ca, high absolute amounts of Cd and Ba and enrichment factors of >1 for Cu, Mn, Ni, Pb, Zn, V, Y, As, Co, Cr, U, Th, La. The samples contain more TOC than average shales, thus the content of trace elements is higher (Table 1;Fig. 4A). Sediment enhance- ment in trace elements due to absorption by organic matter under reducing conditions is probable, assuming that organic matter was reactive (Brumsack, 1980). The appearance of H2S pro-

Fig. 5. Microscopic images of association of microfossils in selected samples (all photographs under normal light, one nicol)

A— radiolarians with juvenile foraminiferids (partly pyritized) in laminated marl sample KP 0; B — trace tests of small planktonic genera within black, laminated shales (JG); C — black cherty shales devoid of microfauna typical of MG samples; D — abundant foraminifera association including epi- and bathypelagic plankton and calcareous benthos — marly sample from JK section; E — radiolarian assemblage within silicified limestone (HL 5)